One problem often encountered in the production of natural gas from underground reservoirs is nitrogen contamination. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. Natural gases which contain a significant amount of nitrogen may not be salable, since they do not meet minimum heating value specifications and/or exceed maximum inert content requirements. As a result, the feed gas will generally be processed to remove heavier components such as natural gas liquids, and then the remaining stream containing primarily nitrogen and methane will be separated cryogenically.
One conventional method of removing the nitrogen contaminant from the natural gas is to pass a stream containing nitrogen and methane to a nitrogen rejection unit (NRU) comprising double cryogenic rectification columns wherein the nitrogen and methane are separated.
Although this conventional method for separating nitrogen and methane has worked reasonably well, a problem related to the nature of rectification has heretofore acted as a detriment to the efficiency of the method.
The problem relates to the fact that the efficiency of the double column cryogenic rectification is hindered at low concentrations of the more volatile component as this reduces the quality of the available reflux for the top of the low pressure column. In the case of a nitrogen-methane mixture, the efficiency of the double-column NRU is significantly reduced when the NRU feed has a nitrogen concentration of less than about 35 mole percent. This results in a significant amount of methane lost in the nitrogen stream exiting the low pressure column. This problem has been addressed by recycling a portion of the nitrogen stream from the NRU separation back to the natural gas feed stream, thus keeping the nitrogen concentration high enough for effective separation. However, this method has two disadvantages. First, use of a nitrogen recycle in this manner increases the NRU plant size reguirements. Second, this process leads to significantly increased power requirements, since relatively pure nitrogen from the exit stream must be separated over again from the natural gas feed.
A recent significant advancement in a double-column NRU process is described in U.S. Pat. No. 4,415,345--Swallow. In this process, a portion of the product nitrogen stream from the low pressure column is rewarmed to ambient temperature, compressed to the pressure level of the high pressure column, and then cooled against the rewarming low pressure nitrogen. This nitrogen stream is then condensed in the high pressure column condenser along with the nitrogen vapor from the high pressure column. By supplementing the amount of nitrogen condensed in this manner, which is often referred to as a nitrogen heat pump, additional nitrogen reflux is available to the low pressure column, thereby permitting a higher percentage recovery of inlet methane. This process has the advantage over the previous state of the art in that a reduction in capital and operating costs is achieved. However, process equipment such as distillation columns and heat exchangers must still be sized for the additional recirculation of nitrogen and a separate nitrogen gas compressor is still required.
Another more recent advancement in such a process is described in U.S. Pat. No. 4,664,686--Pahade. In this process, a stripping column is added to the conventional double column cycle in order to increase the nitrogen concentration of the feed gas to the double column, without requiring nitrogen recompression and recirculation. The addition of the stripping column offers several advantages over the previous state of the art. These advantages include higher methane recovery, decreased operating costs, and increased tolerance to carbon dioxide. However, there is still a significant increase in capital associated with the addition of this stripping column over that of the conventional double column process.
Accordingly, it is an object of this invention to provide an improved process for separating nitrogen and methane.
It is another object of this invention to provide an improved process for separating nitrogen and methane especially when the nitrogen is present in the feed at a concentration not exceeding about 35 mole percent.